The present invention relates to non-contact interferometric measurement of cylindrical objects to determine a variety of parameters and characteristics, e.g. cross-sectional dimensions, velocities, temperature and composition. The invention is directed to several enhancements of devices described in U.S. patent application Ser. No. 08/093,812 entitled "Interferometric Cylinder Sizing and Velocimetry Device" filed Jul. 19, 1993. That application, incorporated herein by reference, is assigned to the assignee of this application.
As in the prior application, the instrument described herein is meant to obtain measurements at high rates while maintaining high accuracy. The enhancements are particularly well suited to the sizing of optical fibers during their manufacture. Since optical fibers are drawn at speeds ranging from 10 to about 100 meters per second, it is necessary to take measurements at high frequency, e.g. up to several hundred thousand measurements per second. This facilitates finding localized defects, confined to a few millimeters of the fiber length. Also, the fiber diameter should be controlled to within about one percent of a selected fiber diameter, to ensure the desired optical and mechanical properties.
As disclosed in the aforementioned application, high speed and accuracy are achieved by detecting, at two different locations, laser energy scattered by the fiber. A variety of manufacturing processes and conditions present challenges in this regard. For example, the phase difference (phase shift) measurement depends upon the fiber refractive index as well as the fiber diameter. The most common cause for a change in refractive index is a change in temperature, although the refractive index also is influenced by a change in material composition. A change in refractive index, if undetected, can lead to an erroneous conclusion as to fiber diameter.
Another challenge, which also is encountered in the related field of laser Doppler velocimetry, is the need to strike a balance between measuring sensitivity (phase shift per incremental diameter change, e.g. degrees/micron) and measurement range. Because of the 360.degree. upper limit to phase shift measurements, increasing sensitivity decreases the measurement range. Otherwise a measured phase may significantly differ from the actual phase shift. This is frequently referred to as "2.pi. ambiguity". A known approach to this problem is to obtain two phase shift measurements, one to accommodate high sensitivity and the other to accommodate a broad range. While this approach is satisfactory in many instances, it is limited by the fact that it offers only one phase difference measurement selected for sensitivity.
Another need arises in connection with enlarging the measuring volume at the intersection of the laser beams. A system with a larger measuring volume requires less alignment skill and thus is easier to use. However, it is subject to variance in detector elevation angles, based on changes in the location at which the fiber or other object intersects the measuring volume. The failure to detect, and compensate for, positional shifts of the fiber can lead to erroneous assumptions as to changes in fiber diameter, caused by phase shift signals that vary according to fiber displacement within the measuring volume.
Yet another challenge arises in connection with a different manufacturing application. More particularly, glass fibers formed in the manufacture of thermal insulation and fiberglass are manufactured using a spinning device that forces molten glass through small openings. The resulting fibers are transported by an airflow. In this process, the requirements of signal rate and fiber sizing accuracy are less demanding than in the fiber drawing process. However, fiber orientation changes significantly with time and signals must be collected intermittently, due to the inevitable flopping of individual fibers.
In view of these challenges, it is an object of the present invention to provide a fiber diameter measurement system incorporating correction for changes in the fiber refractive index and enhanced measurement accuracy through combining two separate measurements of object diameter.
Another object of the invention is to provide an interferometric sizing method that simultaneously affords high sensitivity and a broad measurement range.
A further object is to provide an interferometric cylinder sizing device with a larger measuring volume and compensation for displacements of the measured object within the measuring volume.
Yet another object is to provide an interferometric system for sizing cylindrical objects, with substantially reduced sensitivity to skew of the object relative to the beam plane.